The emptying (blowing) of the digester during cooking in batches takes place in that the contents of the digester are blown out by means of excess pressure to a pulp container. In order that the emptying may be completed quickly and reliably from the process point of view, a relatively high difference in pressure must be maintained between the digester and the pulp container. As a result, released gases and steam, so-called blowing steam, go off from the pulp.
Besides water vapour, the blowing steam contains, inter alia, terpenes, evil-smelling gases such as methyl-mercaptan, dimethyl sulphide, dimethyl disulphide and hydrogen sulphide, as well as nitrogen, carbon monoxide and carbon dioxide. Moreover, several of these gases are poisonous.
The usual method of handling the blowing steam is to cool it down so that the greater part of the condensable gases condense, the steam-forming heat which is released during the condensation being used for the production of hot water which is utilized as far as possible in other processes. The gases which do not condense are greatly contaminated and are generally destroyed by burning.
The disadvantages of this method are that the heat and terpentine content of the blowing steam is partially lost because the production of hot water is greater than the demand in the manufacturing processes and some of the terpenes go off with the non-condensed gases.
The reason why the blowing steam has not been used for preheating of cellulose material before the digester is that the amount of blowing steam which goes off at the beginning of the emptying of a cook is much greater than at the end of the emptying. The pressure in the blowing-steam system varies to a corresponding extent. If the pressure in the system varies much, there is a risk of aftercooking in the pulp container at low pressure in the system, that is to say between two digester blows. A powerful aftercooking leads to cellulose material being drawn into the blowing steam system with the gases where it can cause disturbances in the process.
Between two digester blows, the pressure in the blowing steam system sinks so low that a partial vacuum can form. Cold air is then drawn in, so that cellulose material which has previously been preheated is cooled down again. At the same time, there is a risk of exploding because the mixture of gas and air can be explosive.
The usual method of continuous cooking (sulphate cooking) means that the wood chips from a small chip bin are conveyed to a steaming vessel for preheating with flash steam and low-pressure steam. At the same time, steam and air are extracted from the steaming vessel and conveyed to a terpentine recovery system where the steam is condensed together with the terpentine in the steam. The terpentine is separated in a terpentine decanter.
The wood chips are fed into a digester together with white liquor and the cooking takes place in known manner. A usual method is to wash the cooked cellulose material in the lower part of the digester by extraction counter-currently with a washing liquid. The pulp is discharged at the bottom of the digester and the liquor is taken out as extraction liquor from the upper portion of the washing zone. The temperature of the liquor drops through spontaneous evaporation in two or more pressure expansion stages, or flash cyclones. Steam formed is utilized for heating-up purposes, steaming wood chips in the steaming vessel and for the production of hot water. Usually steam from the first flash stage is used for steaming wood chips in the steaming vessel, while steam from the other flash stage is used for heating up streams of liquor or for the production of hot water.
During recent years, chip bins have begun to be installed for preheating chips with flash steam from the last flash stage or stages to reduce the consumption of steam at high pressure.
It is well known that the recovery of terpentine from installations for continuous sulphate cooking provides a low yield of terpentine. The terpentine is supplied to the process with the wood. Steam and air are extracted from the steaming vessel and are conveyed to a terpentine recovery system, where the steam is condensed. The terpentine which is recovered from the steaming vessel comes partly from the wood chips, partly from the flash steam which is used for the steaming of the chips. Since the terpentine in the chips is very inaccessible and the time it remains in the steaming vessel is short, only a small portion of the terpentine content of the wood can be driven off. Since the extraction steam contains air, some of the terpentine accompanies the air out into the atmosphere after cooling. Thus the fact that the steam contains air reduces the terpentine yield. The greater part of the terpentine follows the wood into the digester and is transferred into liquor during the cook. The terpentine is then driven off during the flashing of the liquor after the digester and during evaporation in an evaporation plant. When this steam condenses, the terpentine also condenses and is present mainly emulsified in condensate from the digestion and evaporation plant.
In connection with the cook, volatile constituents are formed such as evil-smelling sulphur compounds and methanol. They are driven off like the terpentine and are present together with terpentine in certain condensate fractions in the digesion and evaporation plant. Inert gases are also present in the system and are continuously drawn off. These streams also contain terpentine and other volatile, evil-smelling compounds.
The streams of gas containing terpentine and evil-smelling gases are collected and destroyed by burning. Since these vapours mixed with air are explosive, it is important to prevent air from entering the system. As already pointed out earlier, the vapours leaving the terpentine recovery contain both air and terpentine. They therefore constitute a potential safety risk.
The condensate which contains terpentine, evil-smelling compounds and methanol, is likewise collected, conveyed to a distillation column where the volatile compounds are driven out after which they are destroyed by burning. A large proportion of the terpentine supplied is thus burned. At best, therefore, only the fuel value of the terpentine is used. Terpentine is a valuable raw material for chemical production, however, and as such has quite a higher value. It is possible to recover terpentine after the distillation column by cooling and separation. This terpentine contains so many impurities, however, that its value is limited.